The present invention relates generally to alignment techniques and more specifically to an improved visual technique for measuring misalignment between levels on a wafer.
To align a mask relative to a previously formed layer, the mask is adjusted until an indicia pattern on the mask is aligned with the indicia on the substrate formed in the previously formed layer. As explained in U.S. Pat. No. 3,861,798, the accuracy of the alignment technique between the indicia on a mask and the indicia on a substrate is limited by the resolution power of the microscope. Because the two indicia are separated one above the other, the microscope must have a depth of focus large enough to focus both elements at the same time. As the depth of focus of the microscope is increased, the resolution power decreases. Thus, precision in the range of 1 micron is possible for substrate-to-mask separation of 5 to 25 microns. To avoid this problem, U.S. Pat. No. 3,861,798 deals with interference or moire patterns. The technique disclosed therein is where the mask is positioned until the moire patterns produced by the superimposed indicia of the mask and that of the substrate are aligned with additional indicia or indexing bars. Although describing alignment of a mask to a substrate, this patent does not discuss the measurement of misalignment of a first layer relative to a second superimposed layer.
The state of the art for measuring photomask alignment in integrated circuit manufacturing is described in the 1977 IEEE International Electronic Devices Meeting Digest in an article entitled "A Comparison of Electrical and Visual Alignment Test Structures for Evaluating Photomask Alignment in Integrated Circuit Manufacturing", by T. J. Russell, T. F. Leedy, and R. L. Mattis. The optical technique includes a wedge from one mask level superimposed upon a rectangular form from a previous mask level as shown in FIG. 1. The displacement D of the crossing points of the wedge and the triangle is proportional to the misalignment of the two levels. The electrical technique, which was considered superior and is illustrated in FIG. 2, uses the principle of a linear potentiometer to measure the misalignment between the diffusion mask level and the contact aperture mask level. The resistances R.sub.1 and R.sub.2 are determined by measuring the voltage difference between V.sub.2 and V.sub.3, and V.sub.3 and V.sub.4 with currents applied at I through the diffused region. For perfect alignment, R.sub.1 is equal to R.sub.2 ; the misalignment is equal to one half the distance L between V.sub.1 and V.sub.2 times the difference between R.sub.1 and R.sub.2 divided by R. As indicated in the paper, although the electrical technique removes subjective error of the optical technique and is considerably faster, the resolution is not as good.
Thus, there exists a need for a technique which overcomes the problem of the prior art.